The present invention relates to offshore floating platforms, more particularly to a tension leg platform (TLP) for installation in water depths from less than 1,000 to 10,000 ft.
TLPs are floating platforms that are held in place in the ocean by means of vertical structural mooring elements (tendons), which are typically fabricated from high strength, high quality steel tubulars, and include articulated connections on the top and bottom (tendon connectors) that reduce bending moments and stresses in the tendon system. Many factors must be taken into account in designing a TLP to safely transport the TLP to the installation site and keep it safely in place including: (a) limitation of stresses developed in the tendons during extreme storm events and while the TLP is operating in damaged conditions; (b) avoidance of any slackening of tendons and subsequent snap loading or disconnect of tendons as wave troughs and crests pass the TLP hull; (c) allowance for fatigue damage which occurs as a result of the stress cycles in the tendons system throughout its service life; (d) limitation of natural resonance (heave, pitch, roll) motions of the TLP to ensure adequate functional support for personnel, equipment, and risers; (e) maximization of the hydrostatic stability of the TLP during transport and installation; and (e) accommodation of additional requirements allowing for fabrication, transportation, and installation.
These factors have been addressed in the prior art with varying degrees of success. Conventional multi-column TLP's generally have four vertical columns interconnected by pontoons supporting a deck on the upper ends of the vertical columns. Tendons connected at the lower ends of the columns anchor the TLP to the seabed. In such conventional TLP designs, the footprints of the deck, the vertical columns and the tendons are substantially the same and hydrostatic stability of the TLP can be a problem. Some TLP designs address this problem by incorporating pontoons and/or structures that extend outboard of the column(s) to provide a larger tendon footprint to limit natural resonance (heave, pitch, roll) motions of the TLP. In U.S. Pat. No. 6,447,208, a TLP having an extended base substructure is disclosed. Vertical columns supporting a deck on the upper ends thereof form the corners of the substructure. A plurality of wings or arms extends radially out from the outer perimeter of the substructure. The arms increase the radial extension of the base substructure between about 10% and about 100%. The arms include tendon connectors affixed at the distal ends thereof for connection with tendons anchoring the TLP to the seabed. The tendons footprint is substantially larger than the footprint of the substructure.
The present invention, in its various embodiments, addresses the above-described factors to accommodate different payload requirements, various water depths and to improve TLP response. Improvement of TLP performance may be obtained by battering the deck support columns, thereby reducing tendon tension reactions, increasing the free floating stability of the TLP, and reducing overall system costs.